1. Field of the Invention
The present invention relates to a polyester fiber exhibiting excellent dyeing properties at a high temperature, although its normal pressure dyeability is low, and a process for the production as well as a process for the dyeing of the fibrous structure of the polyester fiber.
2. Description of the Related Art
A polyester fiber has a high refractive index among the various synthetic fibers, and is remarkably inferior to an acetate fiber in coloring properties, even when dyed with the same dye concentration. Since, in an extremely fine fiber of polyester, the surface area per a determined weight is increased, and irregular reflections of light (white light) on the fiber surface are increased, it has been said that a dyed product does not exhibit a high color value compared to an ordinary yarn, even if the same amount of dye is used. In order to obtain a dyed product having a deep color, a so-called "high concentration dyeing", becomes necessary in which a dye with a concentration 2 to 6 times as high as the dye concentration for an ordinary yarn is used.
The disadvantages of the high concentration dyeing are as follows:
(A) A large quantity of dye is required, and the utilization efficiency of the dye is lowered, resulting in high production costs.
(B) Many hours are required for dyeing or the cleaning of a dyeing machine.
(C) The fastnesses of a dyed product is lowered.
That is, there are many problems with respect to coloring properties, qualities, workability and production cost, and refinements in these problems are in demand.
With respect to refinement in the dyeability of a polyester fiber, examination of preliminary treatments of the polyester fiber with hot water and a solvent has been proposed by Kuwabara (Sen-i-gakkaishi [Journal of the Institute of Fibers], vol 34, No. 9, p. 56 (1978)), Wakita (Sen-i-gakkaishi, vol. 37, No- 5, p. 33 (1981)), and Katayama ("Senshoku Kogyo" [Dyeing Industry], vol. 24, No. 7, p. 10 (1976)).
In addition, as property modification of a material fiber for making the fiber dyeable under normal or atmospheric pressures, there have been proposed a process for the copolymerization of a polyalkylene glycol with a polyethylene terephthalate in, e.g., Japanese Unexamined Patent Publication No. 52-63292 and Japanese Unexamined Patent Publication No. 54-156861, and a process for the copolymerization of a polyalkylene glycol and isophthalic acid with a polyethylene terephthalate in Japanese Unexamined Patent Publication No. 53-35022.
Furthermore, as a process for spinning a polyethylene terephthalate at a high speed so as to obtain a fiber that is dyeable under normal pressures, there has been proposed, in Japanese Unexamined Patent Publication No. 57-161121, a normal pressure dyeable polyester fiber comprising a high speed spun polyethylene terephthalate fiber with a value of tan .delta..sub.max greater than 0,135 (0,135&lt;tan .delta..sub.max) and a value of T.sub.max (.degree. C.) not higher than 105 (T.sub.max (.degree. C.).ltoreq.105) in a mechanical loss tangent tan .delta.-temperature T curve obtained by a measurement of dynamic viscoelasticity.
In the aforesaid Kuwabara's process, the preliminary treatment is effected at a temperature ranging from 106.degree. C. and 135.degree. C., and the dyeing is effected at a temperature of 105.degree. C. at the highest, and in the aforesaid Wakita's process, the preliminary treatment is effected at a temperature of 140.degree. C. at the highest, and the dyeing is effected at a temperature of 120.degree. C. at the highest. However, a deep color-improving effect for a polyester fiber has not been sufficient according to any of these processes.
In other words, though the aforesaid hitherto known techniques are intended to improve the dyeing properties of a polyester at a low temperature, a notably high dyeability at a high temperature has not been obtained.
On the other hand, Katayama's process has been a process in which a dye is added to a water soluble high boiling medium and dyeing is effected at a high temperature. This process has, however, a defect in the sublimability of the dye and the dyeing fastnesses.
Also with respect to the aforesaid reforming process of a material fiber, when a polyalkylene glycol with a small repeating unit such as polyethylene glycol is copolymerized, the light exposure properties of the material fiber are likely to lower.
In addition, in the high speed spun fiber disclosed in Japanese Unexamined Patent Publication No. 57-161121, that is, a fiber with T.sub.max (.degree. C.).ltoreq.105, T.sub.max of the conventional drawn yarn for clothing within the range between 135 and 140.degree. C., the packing density of a molecular chain belonging to an amorphous region is reduced owing to the lowering of the T.sub.max value and normal pressure dyeability has been obtained, but there is the problem in that the fiber is inferior in characteristics at a high ductility and has a high crystallization degree, which makes it difficult to control the degree of shrinkage and the like, and the utilization sphere of which is therefore narrow.
Accordingly, by the processes of the aforesaid prior art, though normal pressure dyeability may be imparted to a polyester fiber, there have been defects in that the dyeing properties of the fiber at a low temperature cannot be said to be sufficient, that the color fastness is lowered, or that certain properties, such as the mechanical properties of the polyester fiber, are degraded.
In addition, since dyeing of a polyester fiber is generally inferior in coloring properties, it is effected by increasing the concentration of the dye as a measure to counter this tendency. However, the utilization efficiency of the dye cannot be improved by such a measure, and therefore, there have been problems with respect to production costs, as well as drainage.
Further, although it has been recognized that when a polyester fiber is dyed at a temperature of 120.degree. C. or more, adequate dyeing properties can be obtained, the present inventors' examination has proven that especially in the dyeing of an extremely fine fiber, improved dyeing properties become necessary to obtain a deep color.
The aforesaid techniques of improving dyeing properties have been insufficient.